Few people were likely to have been as surprised by last week’s general election in Israel as Prime Minister Benjamin Netanyahu. Mr. Netanyahu thought he went into the ballot from a position of strength and expected a mandate from voters, if not for him and his party, then certainly for the rightwing of the political spectrum where he is firmly rooted.

Instead, Israeli voters divided almost exactly in half, and conservatives even lost four seats from the previous Parliament. It was a stunning rebuke, and one that has potentially profound implications for Israeli society over the long term.

When Mr. Netanyahu called the election, he expected vindication for his hardline views. He ran, as usual, on a national security platform, emphasizing an existential threat posed by Iran’s nuclear program as well as the need to secure Israel’s borders from terrorists exploiting Palestinian unhappiness. Pre-election polls showed his Likud party, along with its coalition partners, retaining a comfortable majority in the next assembly.

Mr. Netanyahu needs new pollsters. Voters split neatly down the middle, returning a new Parliament that gives the conservatives a two-vote edge, 61-59; preliminary results showed a 60-60 divide, but late votes from soldiers, diplomats, prisoners and others who could not go to the polls on Jan. 22 broke the tie. Turnout was stronger than expected: 67 percent of voters, compared with 65 percent in the last election three years ago.

While Mr. Netanyahu’s Likud party remains the largest in the assembly, with 31 seats in partnership with Yisrael Beiteinu, another rightwing party (they are likely to split their seats 20-11), the big winner was Mr. Yair Lapid, a former newscaster who formed the center-left Yesh Atid (“There is a Future”) party just a year ago.

Having won 19 seats, Yesh Atid eclipsed other center-left parties and claimed the second highest presence in Parliament. While sometimes dismissed as a dilettante — his supporters warn that that is a dangerous assumption — Mr. Lapid’s appeal to middle-class secular, and progressive voters captured public sentiment and transformed the political debate.

His platform focused on economic issues, in particular the privileges afforded Israel’s orthodox citizens, such as exemption from military service and special government subsidies. While security concerns remain important, voters worry most about bread and butter issues such as the price of food and housing.

As head of the largest bloc in Parliament, Mr. Netanyahu will get the first chance to cobble together a new government. He could forge a coalition that would just include parties of the right, but he has said he will reach out to Mr. Lapid to form a broad-based government “that is as wide a coalition as possible.”

That strategy makes sense since the conservative-only formula would produce a government dependent on small parties and would alienate half the electorate.

Mr. Lapid is prepared to entertain the idea of a coalition — against the wishes of other left parties which believe the election results are a vote of no-confidence in Mr. Netanyahu — if his programs are adopted. Mr. Netanyahu says that he supports a more equitable “sharing of the burden” — political talk for reducing the privileges accorded religious Jews.

That poses risks to Mr. Netanyahu since one of his traditional allies, the rightwing Shas party, adamantly opposes ending military exemptions for Orthodox Jews. But Shas only holds 11 seats.

Historically, close elections have pushed Israel’s two largest parties together. In the past, the left was represented by Labor. While it has been overtaken by Yesh Atid, it continues to be a force in Israeli politics, having won 15 seats in last week’s ballot.

The chief question for Mr. Netanyahu is whether he reaches across the aisle to Labor for the remaining seats he needs, or will he stick with fellow conservatives, such as Jewish Home, a nationalist party now with 12 seats that opposes Palestinian statehood and wants to annex parts of the West Bank. The party has surged from the three seats it held in the last assembly. Its head, Mr. Naftali Bennett, also supports “sharing of the burden,” but the party’s foreign and security policies may alienate Yesh Atid supporters.

Nevertheless, the smart money is betting that Jewish Home gets a call before Labor (which has said it will not join a Netanyahu-led government). Other small parties, such as the centrist Movement Party, with six seats, could also play a role.

Mr. Netanyahu’s first task will be winning over Mr. Lapid and then seeing what room is left for him to maneuver. While domestic issues were foremost in voters’ minds, security concerns could rise to the forefront if unrest from Syria spills over its borders, Iran becomes more belligerent or Egypt’s government changes its policy toward Israel. Palestinians may also be tempted to exploit this moment and press their case, but that would only strengthen hardliners backing Mr. Netanyahu.

One of the most important factors for the new government will be relations with the United States. That partnership has been testy recently, with Mr. Netanyahu challenging U.S. President Barack Obama and openly backing his challenger Mr. Mitt Romney in the last U.S. presidential election campaign.

While Mr. Obama insists that his support for Israel remains strong, there is no mistaking the shift in the balance of power between the two men. That is another factor Mr. Netanyahu must consider as he forges a new government.